This invention relates to uncooled reflective shields for cryogenically-cooled radiation detectors and, more particularly, to an uncooled reflective shield having multiple segments, each of substantially toroidal shape.
It has been known for some time that proper cold shielding of a high performance infrared detector limits radiation incident upon the detector to that emanating within the optical field of view, thereby improving detectivity, or sensitivity, for the given characteristics of the optical system, object, and detector. The most efficient infrared systems employ a coldshield, which is cooled by a cryogenic cooler, or coldfinger. The coldshield may extend many detector widths away from the detector focal plane, thus establishing an efficient definition of the desired incident ray cone. Ideally, the coldshield would include an acceptance aperture at the exit pupil of the optical system, hereinafter called the entrance pupil of the detector. Unfortunately, such construction has generally required a relatively long and massive shield, cantilevered forward from the detector. This type of shield necessitates the support and cooling of a large physical mass, which increases the difficulty of meeting fast cooldown times and minimum steady-state heat loads.
Previous coldshield designs have a further disadvantage in that optical lens elements between the acceptance aperture of the coldshield and the detector are not readily accommodated, since they complicate the construction of the cold structure. They also tend to increase both the cooldown time and coldfinger deflection at the detector.
In order to reduce the mass of the coldshield, warmshielding has been employed in infrared detector systems. The warmshield gives an intended equivalent shielding result by imaging the detector back upon itself by means of a spherical or elliptical or flat reflector. This type of reflector is not cooled but is left at ambient temperature.
However, the use of the above mentioned types of warmshield geometries gives rise to certain disadvantages. Spherical warmshields are large in diameter and hence are not appropriate for compact infrared detector systems. They also fail to reproduce true coldshield performance since they image the detector, which is an object having a definite and variable reflectance. For example, modern detector arrays may have a finely patterned structure that causes variable geometric reflections from the surface, as well as diffraction scattering and other variable geometric reflection effects. Elliptical warmshields, while generally having a lesser diameter than the spherical warmshields, still are large and, generally, exhibit many of the disadvantages of spherical warmshields. They are also not optimum for use with broad detectors as their imaging performance is poor for off-axis rays. Also, they are susceptible to stray light problems resulting from unwanted forward reflections. Flat warmshields are ineffective for off-axis rays, and are also large in diameter, since the reflected rays often spread to large diameters. Hence, flat warmshields cannot generally be placed as far from the detector as is required for reproducing a true coldshielding effect. Flat warmshields are therefore not generally suitable for use in compact systems.
In addition to their individual problems, all presently utilized geometries of warmshields create a ghost image problem. This problem stems from the imperfect absorption by the detector of imaging rays. Those rays that are not absorbed by the detector will be reflected. When these rays impinge on the reflective surface of the above mentioned types of warmshields, they are reflected once more. Because of the geometries of these types of warmshields, a ray, upon its reflection from the warmshield surface, will be reimaged upon the detector, but at a different point than that at which the ray originally impinged on the detector. Thus, there occurs an apparent doubling of the image viewed by the detector.